The present invention relates to styrenic resin compositions. More specifically, the invention relates to styrenic resin compositions which are heat-sealable.
The dairy industry commonly uses aluminum foil with a hot melt seal lacquer as a lid or sealing structure for many types of plastic containers, such as cups for yogurt and other dairy products. Since aluminum can resist high temperatures, the cycle time for sealing the lid to the container can be kept short. Unfortunately, the tear resistance of aluminum lids is not always sufficient, thereby resulting in tearing of the lid during peeling.
Lids prepared using polyvinyl chloride (PVC) or polyesters, such as PET, are also common in the marketplace. These plastic lids typically employ hot melt coatings to ensure adhesion to the container. The hot melt coatings melt at a temperature lower than the temperature at which the plastic top layer begins to deform or melt and stick to the seal bar. A second type of plastic lid commonly found in the marketplace includes one or more polystyrene-compatible resin layers with one or more polyester top or substrate layers coextruded with the polystyrene-compatible resin layer to improve the strength of the lid, and to prevent the hot-seal bar from sticking to the lid. A disadvantage of lidding systems employing PVC or polyesters is that these systems contain polymers which are not fully compatible with the polystyrene commonly employed in the container to which the lidding system is attached. Therefore, recycling of dairy containers containing diverse polymeric materials requires a preliminary, expensive separation step.
The above-described plastic lid materials are not fully compatible for recycling purposes with polystyrene dairy containers. It would be desirable to have an easily sealed plastic lidding system for thermoformed food containers that peels off easily, while retaining its physical integrity. Integrity is important in the case of food packaging in order to guarantee the shelf life of the packaged food. Additionally, it would be desirable to have a lidding system which is completely compatible with the polystyrene dairy container material, thereby allowing easy recyclability of container and lid without the need for expensive separation steps.
It would be further desirable in the dairy food packaging market to have a composition which would provide a controllable, peelable, adhesive seal after conventional heat sealing processes, which typically last from between 0.5 to 2 seconds, and which would display minimal changes in adhesion force between container and lid with seal bar temperature variations of up to 15.degree. C. Further, it would be desirable if the sealing layer would soften, melt and stick to the container before the lid deforms significantly.